Fix GridFS initial query for documents:
Fix GridFS initial query for documents:

- Negative limit values are invalid on 3.2 and higher.
- Use a projection to only deserialize the _id field.
- Don't convert to array - only need the first document.

jonhyman/mongo-ruby-driver,kay-kim/mongo-ruby-driver,kay-kim/mongo-ruby-driver,brandonblack/mongo-ruby-driver,aherlihy/mongo-ruby-driver,mongodb/mongo-ruby-driver,jonhyman/mongo-ruby-driver,estolfo/mongo-ruby-driver,mongodb/mongo-ruby-driver,saghm/mongo-ruby-driver,saghm/mongo-ruby-driver,wandenberg/mongo-ruby-driver,estolfo/mongo-ruby-driver,mongodb/mongo-ruby-driver